The present invention relates to a method for forming a compound semiconductor layer, and more specifically to a method for forming a group III-V compound semiconductor layer containing at least nitrogen and arsenic as a group V element.
Recently, as group III-V compound semiconductor materials having a significantly wider field of use as optoelectronics materials, group III-V compound semiconductor materials containing arsenic as a group V element (GaAs, GaInAs, etc.) and nitrogen mix-crystallized therewith have been proposed.
Japanese Laid-Open Publication No. 6-37355 (first conventional example) discloses Ga1xe2x88x92yInyNsAs1xe2x88x92x-based compound mix crystal semiconductor materials (z=about 0.04) as new semiconductor materials which are lattice-matched to a GaAs substrate. It is shown that use of such semiconductor materials allows a semiconductor laser for emitting light in a long wavelength band (1.3 to 1.55 xcexcm) to be produced on a low-cost GaAs substrate, which is conventionally impossible.
PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 4, April 1998, page 487 (second conventional example) discloses producing a semiconductor laser structure on a GaAs substrate. The semiconductor laser structure includes an active layer formed of a quantum well layer which is formed of Ga0.7In0.3N0.01As0.99 and a guide layer, and the active layer is held between upper and lower cladding layers formed of Al0.3Ga0.7As. It is reported that such a semiconductor laser realizes continuous oscillation for light having a wavelength of 1.31 xcexcm at room temperature. This is the first report that such a continuous oscillation is realized by a semiconductor laser formed of materials lattice-matched to a GaAs substrate.
For crystal growth of these new semiconductor materials, a molecular beam epitaxy (MBE) method or an metal organic chemical vapor deposition (MOCVD) method is used. Usable nitrogen source materials include, for example, dimethylhydrazine (DMeHy) and nitrogen gas (N2) activated by plasma. Crystal growth is conducted by concurrently supplying Ga, In and As source materials and the nitrogen source material(s) described above.
Such group III-V compound crystal semiconductor materials containing a group III-V compound semiconductor having arsenic as a group V element and also containing nitrogen as a group V element mix-crystallized therewith have not been actively studied until recently. The reason is that it is difficult to grow crystals of such semiconductor materials.
For example, GaAsN is considered to be a mix crystal of GaN containing only N as a group V element and GaAs containing only arsenic as a group V element. This mix crystal system have a very large immiscible region (misciblity gap). Therefore, it is difficult even to introduce only several percent of N with GaAs. Thus, it is necessary to carefully select a method and conditions for crystal growth. It is reported that especially introducing nitrogen with GaAs is significantly influenced by a substrate temperature during crystal growth. As a substrate temperature for such crystal growth, about 500xc2x0 C. is usually selected. The temperature of 500xc2x0 C. is relatively low as a crystal growth temperature of a group III-V compound semiconductor.
Jpn. J. Appl. Phys. Vol. 36, No. 12A, December 1997, page L1572 (third conventional example) shows correlation between the substrate temperature during crystal growth and a nitrogen-mix crystal ratio in the crystal in the case where GaAsN containing monomethylhydrazine (MMeHy) as an N source material in crystal-grown. When the substrate temperature is lower than 500xc2x0 C., MMeHy is not sufficiently thermally decomposed. Therefore, only a small amount of nitrogen is introduced. By contrast, when the substrate temperature is higher than 500xc2x0 C., the nitrogen source material is thermally evaporated significantly, such that nitrogen is not introduced into GaAs. It is reported that N can be introduced into the crystal most efficiently at a substrate temperature of about 500xc2x0 C. for these reasons. In the second conventional example, plasma-decomposed N2 is used as a nitrogen source material. In this example also, about 500xc2x0 C. is selected as a crystal growth temperature.
Novel compound semiconductor materials containing nitrogen mix-crystallized with, for example, GaAs or GaInAs are used for an active layer of a semiconductor laser. One such example is described above, in which a GaInNAs layer is used for an active layer of a semiconductor laser. A semiconductor laser using such a compound semiconductor material does not necessarily provide superior light emission characteristics over an equivalent structure using a compound semiconductor material not containing nitrogen. For example, in the publication showing the above-described second conventional example, semiconductor lasers having structures similar to one another are produced. One of these semiconductor lasers uses GaInAs not containing nitrogen for an active layer (quantum well), and the other semiconductor laser uses GaInNAs containing nitrogen. It is reported that when 1% of nitrogen is contained, the oscillation threshold current becomes four times larger and the light emission efficiency is reduced to about ⅔. It is also reported that when a small amount of nitrogen is contained, the light emission efficiency is drastically reduced.
As one cause of reduction in the light emission efficiency, it can be pointed out that the crystal growth temperature is too low according to the conventional crystal growth method and therefore crystals having sufficient crystallinity are not obtained.
For example, in the case of GaAsN, a crystal is produced by introducing N into GaAs by causing crystal growth to proceed in a state of non-equilibrium at a low growth temperature (about 500xc2x0 C.). Such a crystal cannot be produced in a state of thermal equilibrium. GaAsN can be considered to be a mix crystal of GaAs and GaN. The optimum growth temperature of GaAs is 600xc2x0 C. to 750xc2x0 C., and the optimum growth temperature of GaN is 900 to 1000xc2x0 C. As compared to these temperatures, about 500xc2x0 C. cannot be considered to be the optimum growth temperature for GaAsN-based compound mix crystal semiconductor materials.
It is assumed, for example, that in a semiconductor laser including an active layer and upper and lower cladding layers sandwiching the active layer, the active layer is formed of GaInNAs and the upper and lower cladding layers are formed of, for example, AlGaAs, GaInP, InGaAsP or AlGaInP. For producing such a semiconductor laser, the crystal growth temperature for the upper and lower cladding layers formed of AlhGaiIn1xe2x88x92hxe2x88x92iAsjP1xe2x88x92j (hxe2x89xa70,i greater than 0,jxe2x89xa70) is usually set to be a low substrate temperature (about 500xc2x0 C.) in conformity with the crystal growth temperature for the GaInNAs active layer. As described above, the cladding layers crystal-grown at such a low substrate temperature do not have sufficient crystallinity. Unless the lower cladding layer formed of AlhGaiIn1xe2x88x92hxe2x88x92iAsjP1xe2x88x92j (hxe2x89xa70,i  greater than 0, jxe2x89xa70) which acts as an underlying layer of the GaInNAs active layer has sufficient crystallinity, the crystal defect of the lower cladding layer is transferred to the GaInNAs active layer which is crystal-grown on the lower cladding layer. Accordingly, when a laser structure is produced at such a low temperature, satisfactory light emission characteristics cannot be provided, and the laser device deteriorates quickly. Such a conventional low temperature crystal growth method is considered to be performed in order to meet a requirement for provision of a novel material by introducing nitrogen rather than a requirement for improvement in the light emission characteristics by growth of GnAsN or GaInNAs at a high temperature.
As an attempt to improve light emission characteristics, there is a report on the effect of heat treatment performed after the crystal growth. The abstract of Jpn. J. Appl. Phys. Spring 1998, 28p-ZM-12 reports that the light emission intensity becomes 25 times higher by heating GaAsN (nitrogen composition ratio: 0.79%) at 700xc2x0 C. for 10 minutes in a hydrogen atmosphere. However, the studies of the present inventors found that the laser characteristics of a semiconductor laser using GaInNAs for an active layer cannot be improved to a practically usable level by merely performing heat treatment after the crystal growth. The studies of the present inventor also found that means for providing a satisfactory crystal structure during crystal growth is necessary instead of such a treatment performed after the crystal growth.
The present invention has an objective of solving the above-described problems. Namely, it is an objective of the present invention to provide a method for forming a group III-V compound semiconductor layer containing a group III-V compound semiconductor having arsenic as a group V element and also containing nitrogen mix-crystallized therewith, the group III-V compound semiconductor layer having satisfactory light emission characteristics.
A method for forming a compound semiconductor layer according to the present invention includes the step of crystal-growing a group III-V compound semiconductor layer containing at least nitrogen and arsenic as group V elements on a single crystal substrate. The step of crystal-growing the compound semiconductor layer includes the step of supplying a nitrogen source material to the single crystal substrate so that the nitrogen source material interacts with aluminum at least on a crystal growth surface of the compound semiconductor layer.
In another aspect of the invention, a method for forming a compound semiconductor layer according to the present invention includes the step of crystal-growing a group III-V compound semiconductor layer containing at least nitrogen and arsenic as group V elements on a single crystal substrate. The step of crystal-growing the compound semiconductor layer includes the step of supplying an aluminum source material to the single crystal substrate concurrently with a nitrogen source material.
In still another aspect of the invention, a method for forming a compound semiconductor layer according to the present invention includes the step of crystal-growing a group III-V compound semiconductor layer containing at least nitrogen and arsenic as group V elements on a single crystal substrate. The step of crystal-growing the compound semiconductor layer includes the step of supplying a nitrogen source material to a crystal surface of the compound semiconductor layer in a state where the group III atoms containing aluminum are exposed to the crystal surface.
In one preferable embodiment, an aluminum-mix crystal ratio in a group III element in the compound semiconductor layer in 0.02 or higher.
In one preferable embodiment, the step of crystal-growing the compound semiconductor layer is performed at a temperature of the single crystal substrate in the range of 500xc2x0 C. or higher and 750xc2x0 C. or lower, and more preferably in the range of 600xc2x0 C. or higher and 750xc2x0 C. or lower.
In one preferable embodiment, the nitrogen source material contains 
where R1, R2, R3 and R4 are hydrogen or a lower alkyl group.
In one preferable embodiment, more than 0% and less than 50% of the crystal growth surface of the compound semiconductor layer is covered with group V atoms.
In one preferable embodiment, the step of crystal-growing the compound semiconductor layer further includes the step of supplying a group III source material containing aluminum and the step of supplying an arsenic source material, wherein a process sequentially including the step of supplying the group III source material, the step of supplying the nitrogen source material, and the step of supplying the arsenic source material is performed at least once.
In one preferable embodiment, the single crystal substrate has a {100} plane as a principal plane.
In one preferable embodiment, the step of crystal-growing a layer formed of AlhGaiIn1xe2x88x92hxe2x88x92iAsjP1xe2x88x92j (hxe2x89xa70, i greater than 0, jxe2x89xa70) on the single crystal substrate is further included. The step of crystal-growing the compound semiconductor layer and the step of growing the crystal formed of AlhGaiIn1xe2x88x92hxe2x88x92iAsjP1xe2x88x92j are performed at the same temperature.
In one preferable, embodiment, the step of crystal-growing the compound semiconductor layer is performed after the step of crystal-growing the layer formed of AlhGaiIn1xe2x88x92hxe2x88x92iAsjP1xe2x88x92j.
In one preferable embodiment, the step of crystal-growing the compound semiconductor layer is performed before the step of crystal-growing the layer formed of AlhGaiIn1xe2x88x92hxe2x88x92iAsjP1xe2x88x92j.
In one preferable embodiment, the compound semiconductor layer further contains indium.
A compound semiconductor apparatus according to the present invention includes at least one group III-V compound semiconductor layer containing at least nitrogen and arsenic as group V elements. The compound semiconductor layer is formed in accordance with any one of the methods for forming a compound semiconductor layer described above.
In one preferable embodiment, the compound semiconductor apparatus in a light emitting device including at least a light emitting layer, and the light emitting layer includes the compound semiconductor layer.
In one preferable embodiment, the light emitting layer is formed of AlxGayIn1xe2x88x92xxe2x88x92yNxAs1xe2x88x92x (0. less than x, y, z less than 1), and an Al-mix crystal ratio x in the light emitting layer is 0.02 or higher and 0.20 or lower, and more preferably 0.02 or higher and 0.10 or lower.
In one preferable embodiment, the light emitting device further includes a cladding layer, a guide layer and/or a barrier layer formed of AlhGaiIn1xe2x88x92hxe2x88x92iAsjP1xe2x88x92j (hxe2x89xa70, i greater than 0, jxe2x89xa70).
Hereinafter, the function of the present invention will be described.
According to the present invention, a nitrogen source material is supplied to a single crystal substrate so that the nitrogen source material interacts with aluminum at least on a crystal growth surface of a compound semiconductor layer. Therefore, a decomposition reaction of the nitrogen source material on the surface of the substrate is promoted, and thus thermal evaporation of nitrogen is suppressed. Consequently, even when the crystal growth temperature is raised to be relatively high (600xc2x0 C. or higher and 750xc2x0 C. or lower), a sufficient amount of nitrogen is introduced. As a result, a crystal having satisfactory crystallinity and especially satisfactory light emission characteristics can be provided. A crystal formed of AlGaAs, GaInP, InGaAsP or AlGaInP has satisfactory crystallinity as a result of being crystal-grown at a relatively high temperature which is equal to the above crystal growth temperature. Accordingly, for producing a multi-layer film containing such a crystal layer on at least one surface of a group III-V compound semiconductor crystal layer, the growth temperature of the group III-V compound semiconductor crystal can be adjusted to be in a temperature range which is optimum for the above-described materials. Thus, the growth temperature of the multi-layer film can be maintained so as to be high. Therefore, a hetero junction of high quality crystals can be formed.